The present invention relates generally to the power regulating and electrostatographic printing arts. More particularly, the invention concerns a rms voltage controller for ensuring constant power dissipation by a fixed load regardless of variations in the line input voltage. In a preferred form, a controller in accordance with the invention is advantageously employed to control the rms voltage supplied to a fusing apparatus of an electrostatographic printing machine.
In the process of xerography, an exemplary form of electrostatographic printing, heat is applied to permanently affix powder toner images to a variety of support surfaces, such as individual copy sheets. This process of applying heat is conventionally referred to as fusing and is carried out by a fusing apparatus, or simply a fuser. A resistance element, such as a lamp, is typically employed to generate the heat necessary for the fusing process.
To maintain a consistent level of copy quality, it is necessary to maintain the temperature of the fuser within a critical tolerance range. If the fuser temperature is too low, fusing of the powder images may be incomplete, producing smeared or incompletely copied final images. Fuser temperatures which are too high raise the likelihood that the copy sheets may scorch or burn. The sources to which printing machines are connected, typically 115 volts AC, exhibit inevitable variations in the line voltage supplied. In recognition of these voltage fluctuations, a variety of regulating devices have been heretofore developed.
For instance, it is known in the prior art to control the power input to the fuser in response to voltage levels across the fuser heat source. U.S. Pat. No. 3,881,085 to Traister, discloses a fuser control circuit in which a switching means, such as a silicon controlled rectifier is triggered to interrupt power to the fuser heating source when a preset level of line voltage is detected across the heating element. Separate R/C circuitry is used to set and reset an amplifier to selectively inhibit the silicon controlled rectifier and thus interrupt power supply to the heating element.
Another prior art control system is shown in U.S. Pat. No. 3,735,092 to Traister. A thermistor senses changes in the fuser temperature, providing a signal which controls a switching amplifier. When a normal operating temperature is attained in the fuser, the switching amplifier is triggered to a non-conducting state which opens a switch to interrupt power to the fuser heating element.
Another known class of regulating device seeks to maintain a constant power input to the fuser. In U.S. Pat. No. 3,961,236 to Rodek et al, for example, constant power regulation is sought by monitoring both the voltage across the fuser load and the current therethrough. A summation of the detected load voltage and current provides an approximation of the power consumption which is utilized to control the power input to the fuser. To effect the desired control, a triac is selectively gated, i.e. triggered on and off, to inhibit the supply of power from the source to the fuser circuitry, the triggering being effected at zero crossing points of the supply voltage waveform for predetermined numbers of half cycles.
Another illustrative circuit for regulating the power applied to a load by controlling the number of cycles of supplied voltage is shown in U.S. Pat. No. 3,579,096 to Buchanan. U.S. Pat. No. 4,223,207 to Chow discloses a circuit for controlling the power supplied to a load by varying the duty cycle of the AC signal supplied to the load.
Other known control systems have been developed to regulate rms voltage across a fuser element. Since it may generally be assumed that the resistance of the fuser element will not change appreciably, it follows that control of the rms voltage across the load will effectively control the power dissipated thereby. In one such controller, a digital signal equivalent of a sample of the fuser input voltage is supplied to a processor. In response to the digitized signal, the processor selectively gates the input voltage source across the fuser heating element in accordance with a plurality of gate activation rates stored in a register associated with the processor.
The foregoing controllers are either costly or do not optimally deliver accurate, precise, control of power supplied to the load. A characteristic problem with the controllers which function to periodically inhibit or suppress full or half cycles of the applied waveform is the inability of the control circuitry to accurately determine when a sufficient number of cycles have been conducted to warrant interruption of the delivery of voltage to the load. The present invention is primarily directed to alleviation of this problem.